ERS for Stabilizing tannery waste SLUDGE
ERS for Stabilizing tannery waste SLUDGE
BACKGROUND
- Tannery industries create serious environmental problems especially in terms of polluting organic effluent and hazardous solid waste as a result of hides and skin processing.
- The industry generates a significant quantity of solid wastes (0.7kg/kg of hides or skins processed).
- The process of tanning consists of the transformation of animal skin to leather.
- Animal skin (cow, goat, sheep and other animals) is submitted to different processes to eliminate meat, fat and hair in which different chemicals such as sodium hydroxide, sodium hypochlorite, enzymes, lime, chlorides, sulfuric acid, formic acid, ammonium salts, kerosene, chlorobenzene, tenso-active reagents, and other compounds are used. The obtained hide is than treated with Cr3þ or tannins, mineral salts and colours to obtain leather.
Leather Industry of India
- The annual global leather trade is estimated to be worth $159.89 billion with the Indian share being approximately 3%. The annual export business of this sector for India is approximately $10 billion and its leather industry is the sixth largest in the world.
- India accounts for 10% (235 million pieces, two billion square feet of leather) of global supplies of raw skins and hides.
- There are around 3,000 tanneries in India (75% in the small-scale sector) and around 50MLD (million litres a day) liquid waste and 305 million kilograms of solid waste a day is generated.
Solid Wastes in Tannery sector
- Fleshing and sludge are the two major solid wastes emanating from tanning and treatment of tannery wastewater. It is reported that about 140-200 kg of fleshing are generated for every ton of leather processed.
- It contains about 80-90% moisture, 6-12% dry volatile matter and 4-8% ash and minerals.
- Effluents from leather processing contain large concentrations of inorganic nitrogen (N) and N-rich organic residues. Apart from organic material, which releases valuable nutrients on decomposition, tannery effluent may contain chromium (Cr), pathogens, mainly of faecal origin, and toxic organic components, all of which pose a serious threat to the environment. If not adequately treated, wastewater from tanneries contaminates surface water and sediments to an unacceptable level.
Tannery Sludge Management
- The CETPs employ conventional and well established physico-chemical, biological treatment technologies, which helps to remove the contaminants such as organics, heavy metals, dissolved and suspended solids.
- The effluent once collected is pumped to tanks to be treated through the reverse osmosis (RO) process.
- Residual water is pumped back to the tanneries for reuse. Part of the sludge that settles during the RO process is pumped to sludge drying beds, while the rest goes to the filter press for dewatering.
- The dewatered sludge is deposited in Secure Land Fill (SLF) tanks.
- Regulation states that only dewatered sludge is to be stored in these tanks.
Landfilling the HAZARDOUS Sludge
- Landfilling in ordinary or special landfills remains the prevailing disposal method. Regrettably, in India, properly designed, constructed and/or maintained landfills are not available; tannery sludge is just dumped without any control.
- The current management of these solid wastes triggers some secondary and tertiary environmental impacts.
- The environmental problems in the tanning industry have become more challenging after the Supreme Court verdict leading to the closure of polluting tanneries during 1995.
Treatment of ‘Heavy Metals’ by ERS
- Microorganisms like bacteria, fungi and yeast are known to tolerate and accumulate heavy metals. The microorganisms able to absorb and detoxify heavy metals shall be present in ERS for treatment of metal-contaminated wastewater and that could be expected to immobilize metals in solid waste.
- The disposal of tannery waste on land as a fertilizer is associated with environmental problems due to the presence of heavy metals. During the composting process, the metals content is reduced by ERS microbes. The inoculation of microorganisms in ERS could be very useful to improve the composting process by enhancing enzymatic activity and quality level of the compost which shall be acceptable, with very low heavy metal content.
Removal of CHROMIUM by ERS from ‘Treatment Sludge’ of Tanneries
- Government of India has issued a notification (January, 2000) deleting the tannery sludge from the list of hazardous wastes if the level of trivalent chromium in sludge is less than 5000 mg/kg (dry weight basis), and hexavalent chromium, less than 50 mg/kg.
- This has made COMPOSTING of sludge by ERS an interesting option for low-chrome Treated sludge, especially from tanneries that process semi-processed hides / skins to finished leather.
- Chromium Sludge has trivalent and hexavalent chromium. Hexavalent Chromium is much more toxic compared to trivalent. The bacterial strains used in ERS can reduce hexavalent chromium to trivalent chromium. The latter is less toxic and paves the way for easy absorption. It will not leach from soil. Thus water gets cleared from hexavalent and trivalent chromium.
The ERS option offers a promising avenue for utilisation of sludge containing trivalent chrome at less than 5000 mg/kg and meeting the other disposal standards specified.
Sludge Treatment in ERS
- The sludge waste of 98% moisture content is put directly into ERS.
- Water content of sludge cab be reduced to 20% contents with speed drying.
- ERS Substantially reduced the volume hence save transportation cost.
- Land remedial usage also possible
Practical Application of ERS in ‘Dredging Sludge Treatment’
- Silt sludge becomes aggregated structure with microbe.
- Volume reduction of silt sludge
- Processed silt sludge can be recycled